1. Field of the Invention
The present disclosure relates to a method of performing cell measurement and a method of providing information for cell measurement.
2. Discussion of the Related Art
Recently, studies on a next-generation multimedia radio communication system have been actively conducted. The radio communication system requires a system that can process various information including images, radio data, etc. in lieu of services mainly using voice and transmit the information. The object of the radio communication system enables a plurality of users to perform reliable communication regardless of location and mobility. However, wireless channels suffer from several problems such as path loss, shadowing, fading, noise, limited bandwidth, power limitation of terminals and inter-user interference. Other challenges in the design of the radio communication system include resource allocation, mobility issues related to rapidly changing physical channels, portability and design for providing security and privacy.
When a transmission channel suffers from deep fading, if another version or replica of a signal transmitted to a receiver is not separately transmitted to the receiver, it is difficult for a receiver to determine the transmitted signal. A resource corresponding the separate version or replica is called as a diversity, and the diversity is one of the most important factors contributing to reliable transmission. If the transmission capacity or transmission reliability of data can be maximized using the diversity, and a system for implementing a diversity using multiple transmit and receive antennas is referred to as a multiple input multiple output (MIMO) system.
Techniques for implementing the diversity in the MIMO system are space frequency block code (SFBC), space time block code (STBC), cyclic delay diversity (CDD), frequency switched transmit diversity (FSTD), time switched transmit diversity (TSTD), precoding vector switching (PVS), spatial multiplexing (SM), etc.
Meanwhile, one of systems considered after the 3rd generation system is an orthogonal frequency division multiplexing (OFDM) system capable of reducing an inter-symbol interference effect with low complexity. The OFDM system converts serially input data into N parallel data and transmits the N parallel data respectively carried by N orthogonal subcarriers. The subcarrier maintains orthogonality in terms of frequencies. Orthogonal frequency division multiple access (OFDMA) refers to Orthogonal Frequency Division Multiple Access (OFDMA) refers to a multiple access method of realizing multi-access by independently providing users with some of available subcarriers in a system using OFDM as a modulation method.
FIG. 1 illustrates a radio communication system.
Referring to FIG. 1, the radio communication system includes at least one base station (BS) 20. Each of the BSs 20 provides a communication service for a specific terrestrial area (generally, referred to as a cell) 20a, 20b or 20c. The cell may be divided into a plurality of areas (also referred to as sectors). A user equipment (UE) 10 may be fixed or have mobility. The UE 10 may be called as other terms including a mobile station (MS), a subscriber station (SS), a wireless device, a personal digital assistant (PDA), a wireless modem, a handheld device, etc. The BS 20 generally refers to a fixed station communicating with the UEs 10, and may be called as other terms including an evolved-NodeB (eNB), a base transceiver system, an access point, etc.
Hereinafter, downlink (DL) means communication from a BS to a UE, and uplink (UL) means communication from a UE to a BS. In the DL, a transmitter may be a portion of the BS and a receiver may be a portion of the UE. In the UL, a transmitter may be a portion of the UE and a receiver may be a portion of the BS.
The radio communication system may be any one of a multiple input multiple output (MIMO) system, a multiple input single output (MISO) system, a single input single output (SISO) system and a single input multiple output (SIMO). The MIMO system uses a plurality of transmit antennas and a plurality of receive antenna. The MISO system uses a plurality of transmit antennas and one receive antenna. The SISO system uses one transmit antenna and one receive antenna. The SIMO system uses one transmit antenna and a plurality of receive antennas.
Hereinafter, the transmit antenna means a physical or logical antenna used to transmit one signal or stream, and the receive antenna means a physical or logical antenna used to receive one signal or stream.
Meanwhile, a long term evolution (LTE) system defined by 3rd generation partnership project (3GPP) employs the MIMO. Hereinafter, the LTE system will be described in detail.
FIG. 2 illustrates a structure of a radio frame in 3GPP LTE.
Referring to FIG. 2, the radio frame is composed of ten subframes, and one subframe is composed of two slots. The slots in the radio frame are designated by slot numbers from 0 to 19. The time at which one subframe is transmitted is referred to as a transmission time interval (TTI). The TTI may be called as a scheduling unit for data transmission. For example, the length of one radio frame may be 10 ms, the length of one subframe may be 1 ms, and the length of one slot may be 0.5 ms.
The structure of the radio frame is merely an example, and the number of subframes included in the radio frame, the number of slots included in the subframe, etc. may be variously modified.
FIG. 3 is an exemplary view illustrating a resource grid for one UL slot in the 3GPP LTE.
Referring to FIG. 3, the UL slot includes a plurality of OFDM symbols in a time domain, and includes NUL resource blocks (RBs) in a frequency domain. The OFDM symbol is used to represent one symbol period, may be called as an SC-FDMA symbol, OFDMA symbol or symbol period depending on a system. The BS includes a plurality of subcarriers in the frequency domain as a resource allocation unit. The number NUL of RBs included in the UL slot depends on the UL transmission bandwidth configured in a cell. Each element on a resource grid is referred to as a resource element.
Although it has been illustrated in FIG. 3 that one RB includes a 712 resource element composed of 7 OFDM symbols in the time domain and 12 subcarriers in the frequency domain, the number of subcarriers and the number of OFDM symbols in the RB are not limited thereto. The number of OFDM symbols and the number of subcarriers in the RB may be variously changed. The number of OFDM symbols may be changed depending on the length of a cyclic prefix (CP). For example, the number of OFDM symbols in a normal CP is 7, and the number of OFDM symbols in an extended CP is 6.
The resource grid for one UL slot in the 3GPP LTE of FIG. 3 may be applied to the resource grid for one DL slot.
FIG. 4 illustrates a structure of a DL subframe.
The DL subframe includes two slots in the time domain, and each of the slots includes seven OFDM symbols in the normal CP. Maximum three OFDM symbols (maximum four OFDM symbols for a bandwidth of 1.4 MHz) prior to a first slot in the subframe become a control region to which control channels are allocated, and the other OFDM symbols become a data region to which a downlink shared channel (PDSCH) is allocated. The PDSCH means a channel through which a BS transmits data to a UE.
A physical downlink control channel (PDCCH) may carry resource allocation (also referred to as DL grant) and transmission format on a downlink-shared channel (DL-SCH), resource allocation information (also referred to as UL grant) on a uplink-shared channel (UL-SCH), paging information on a paging channel (PCH), system information on the DL-SCH, resource allocation of an upper layer control message such as a random access response transmitted on the PDSCH, a set of transmission power control (TPC) for individual UEs in a UE group, activation of a voice over Internet protocol (VoIP), etc. The control information transmitted through the PDCCH as described above is referred as downlink control information (DCI).
Hereinafter, a downlink reference signal will be described in detail.
In the 3GPP LTE system, two kinds of DL reference signals, i.e., a common reference signal (RS) or cell-specific RS (CRS) and a dedicated RS or UE-specific RS (DRS) are defined so as to provide a unicast service.
The common RS is a reference signal shared by all UEs in a cell, and is used to obtain information on a channel state and perform handover measurement. The dedicated RS is a reference signal for only a specific UE, and is used to perform data demodulation. The CRS is a cell-specific reference signal, and DRS is a UE-specific reference signal.
The UE measures a common RS and informs the BS of feedback information such as channel quality information (CQI), precoding matrix indicator (PMI) and rank indicator (RI). The BS performs DL frequency domain scheduling using the feedback information received from the UE.
To transmit an RS to the UE, the BS allocates a resource in consideration of the amount of radio resource to be allocated to the RS, the exclusive position of the RS and the dedicated RS, the position of a synchronization channel (SCH) and a broadcast channel (BCH), the density of the dedicated RS, etc.
If a relatively large quantity of resource is allocated for the RS, it is possible to obtain a high channel estimation performance, but a data transmission rate is decreased. If a relatively small quantity of resource is allocated for the RS, it is possible to obtain a high data transmission rate, but the channel estimation performance may be degraded due to a low density of the RS.
Meanwhile, in the 3GPP LTE system, the DRS is used only for data demodulation, and the CRS are used for both objects of channel information acquisition and data demodulation. Particularly, the CRS is transmitted every subframe in a broad band, and is transmitted for each antenna port of the BS. For example, when the number of transmit antennas of the BS is two, CRSs for antenna ports 0 and 1 are transmitted. When the number of transmit antennas of the BS is four, CRSs for antenna ports 0 to 3 are transmitted.
FIG. 5 illustrates an example of the structure of the uplink subframe in the 3GPP LTE.
Referring to FIG. 5, the uplink subframe may be divided into a control region in which a physical uplink control channel (PUCCH) carrying uplink control information is allocated and a data region in which a physical uplink shared channel (PUSCH) carrying uplink data information is allocated. To maintain a single carrier property, RSs allocated to one UE are contiguous in the frequency domain. The one UE cannot transmit the PUCCH and the PUSCH at the same time.
The PUCCH for one UE is allocated as an RB pair in a subframe. RBs constituting the RB pair occupy different subcarriers in first and second slots, respectively. The frequency occupied by each of the RBs constituting the RB pair is changed at a boundary between the slots. The UE transmits uplink control information through different subcarriers according to time, thereby obtaining a frequency diversity gain.
The uplink control information transmitted on the PUCCH includes hybrid automatic repeat request (HARQ) acknowledgement (ACK)/negative acknowledgement (NACK), channel quality indicator indicating a downlink channel state, scheduling request (SR) that is an uplink radio resource allocation request, etc.
The PUSCH is mapped to the UL-SCH that is a transport channel. Uplink data transmitted on the PUSCH may be a transport block that is a data block for the UL-SCH transmitted for the TTI. The transport block may be user information, or the uplink data may be multiplexed data. The multiplexed data may be data obtained by multiplexing the transport block for the UL-SCH and control information. For example, the control information multiplexed by the data may include CQI, PMI, HARQ ACK/NACK, RI, etc. The uplink data may be composed of only the control information.
Meanwhile, a high data transmission rate is required, and the most basic and stable plan for solving the high data transmission rate is to increase a bandwidth.
However, frequency resources are currently in a saturation state, various technologies are partially used in a wide frequency band. For this reason, carrier aggregation (CA) has been introduced as a plan for securing a wideband bandwidth in order to satisfy the requirement of the high data transmission rate. Here, the CA is a concept of designing to satisfy basic requirements that an independence system is operable in each of the scattered bands and binding a plurality of bands using one system. In the CA, the band in which the independent system is operable is defined as a component carrier (CC).
The CA is employed not only in an LTE system but also in an LTE-advanced (hereinafter, referred to as an ‘LTE-A’).
Carrier Aggregation
A carrier aggregation system refers to a system that forms a wide band by aggregating one or more carriers having a bandwidth narrower than a desired wideband when a radio communication system intends to support the wideband. The carrier aggregation system may be called as other terms including a multiple carrier system, a bandwidth aggregation system, etc. The carrier aggregation system may be divided into a contiguous carrier aggregation system in which carriers are contiguous and a non-contiguous carrier aggregation system in which carriers are separated from one another. Hereinafter, when the carrier aggregation system is simply called as a multiple carrier system or carrier aggregation system, it should be understood that the carrier aggregation system includes both cases in which component carriers are contiguous and in which component carriers are non-contiguous.
In the contiguous carrier aggregation system, a guard band may exist between carriers. When one or more carriers are aggregated, the carriers to be aggregated may use the bandwidth used in a conventional system as it is for the purpose of backward compatibility with the conventional system. For example, the 3GPP LTE system supports bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. Alternatively, the 3GPP LTE does not use the bandwidth used in the conventional system as it is but may form a wideband by defining a new bandwidth.
In the carrier aggregation system, the UE may simultaneously transmit or receive one or a plurality of carriers according to its capacity.
FIG. 6 illustrates an example of performing communication under a single component carrier situation. FIG. 6 may correspond to an example of performing communication in an LTE system.
Referring to FIG. 6, a general frequency division duplex (FDD) radio communication system transmits/receives data through one downlink band and one uplink band corresponding thereto. The BS and the UE transmits/receive data and/or control information scheduled as a subframe unit. The data is transmitted/received through the data region configured in the uplink/downlink subframe, and the control information is transmitted/received through the control region configured in the uplink/downlink subframe. To this end, the uplink/downlink subframe carries signals through various physical channels. Although the FDD radio communication system has been mainly described in FIG. 6, the aforementioned description may be applied to a time division duplex (TDD) radio communication system by dividing a radio frame into uplink/downlink radio frames in the time domain.
FIG. 7 illustrates an example of performing communication under a multiple component carrier situation. FIG. 7 may correspond to an example of performing communication in an LET-A system.
The LTE-A system uses a carrier aggregation, bandwidth aggregation or spectrum aggregation using a wider uplink/downlink bandwidth by aggregating a plurality of uplink/downlink frequency blocks so as to use a wider frequency band. Each of the frequency blocks is transmitted using a component carrier (CC). In this specification, the CC may mean a frequency block for carrier aggregation or a central carrier of the frequency block according to the context, and the frequency block and the central carrier are used together.
On the other hand, the 3GPP LTE system supports a case in which the uplink/downlink bandwidths are configured differently, but supports one CC in each of the uplink/downlink bandwidths. The 3GPP LTE system supports a maximum bandwidth of 20 MHz, and supports only one CC in each of the uplink/downlink bandwidths. Here, the uplink/downlink bandwidths may be different from each other.
However, the spectrum aggregation (bandwidth aggregation or carrier aggregation) supports a plurality of CCs. For example, if five CCs are allocated as the granularity of a carrier unit having a bandwidth of 20 MHz, the spectrum aggregation can support a maximum bandwidth of 100 MHz.
A pair of DL CC or UL CC and DL CC may correspond to one cell. The one cell basically includes one DL CC and optionally includes UL CC. Therefore, it may be considered that the UE communicating with the BS through a plurality of DL CCs receive services from a plurality of serving cells. The DL is composed of a plurality of DL CCs, but the UL may use only one CC. In this case, it may be considered that the UE receives services from a plurality of serving cells in the DL and receives a service from one serving cell in the UL.
In this meaning, the serving cell may be divided into a primary cell and a secondary cell. The primary cell operates at a primary frequency, and is used to perform an initial connection establishment process, connection re-establishment process or handover process of the UE. The primary cell is also referred to as a reference cell. The secondary cell operates at a secondary frequency, and may be configured after RRC connection is established. The secondary cell may be used to provide an additional radio resource. At least one primary cell is always configured, and the secondary cell may be added/modified/cancelled by upper layer signaling (e.g., an RRC message).
Referring to FIG. 7, five CCs having a bandwidth of 20 MHz may be aggregated in each of the UL/DL, thereby supporting a bandwidth of 100 MHz. CCs may be adjacent or non-adjacent to one another in the frequency domain. For convenience, FIG. 9 illustrates a case in which the bandwidths of UL and DL CCs are identical and symmetric to each other. However, the bandwidth of each of the CCs may be independently determined. For example, the bandwidth of the UL CC may be configured as 5 MHz (UL CC0)+20 MHz (UL CC1)+20 MHz (UL CC2)+20 MHz (UL CC3)+5 MHz (UL CC4). Asymmetric carrier aggregation may be implemented in which the number of UL CCs is different from that of DL CCs. The asymmetric carrier aggregation may be formed due to limitation of an available frequency band or may be artificially formed by network configuration. For example, although the frequency band of the entire system is composed of N CCs, the frequency band received by a specific UE may be limited to M (<N) CCs. Various parameters for the CA may be configured in a cell-specific, UE group-specific or UE-specific manner.
Although it has been illustrated in FIG. 7 that the UL and DL signals are respectively transmitted through CCs mapped one by one, the CC through which a signal is substantially transmitted may be changed depending on the network configuration or kind of signal.
For example, when a scheduling command is downlink-transmitted through the DL CC1, data according to the scheduling data may be transmitted through another DL CC or UL CC. Control information related on the DL CC may be uplink-transmitted through a specific UL CC regardless of the presence of mapping. Similarly, DL control information may also be transmitted through a specific DL CC.
FIG. 8 is a block diagram illustrating a single carrier-frequency division multiple access (SC-FDMA) transmission scheme that is an uplink access scheme employed in the 3GPP LTE.
SC-FDMA is employed in the uplink of LTE. Here, the SC-FDMA is a scheme similar to OFDM, but can reduce power consumption of a portable terminal and cost of a power amplifier by decreasing a peak to average power ratio (PAPR).
The SC-FDMA is a scheme similar to the OFDM in which a signal is divided into sub-bands to be transmitted through sub-carriers using fast Fourier transform (FFT) and inverse-FFT (IFFT). The SC-FDMA is identical to the conventional OFDM scheme in that a guard interval (cyclic prefix) is used so that it is possible to utilize a simple equalizer in the frequency domain with respect to inter-symbol interference (ISI). However, the power efficiency of a transmitter has been improved by decreasing the PAPR at a transmitter terminal by about 2 to 3 dB using an additional unique technique.
That is, the problem of the conventional OFDM receiver is that signals carried by each sub-carrier on a frequency axis are converted into signals on a time axis by the IFFT. Since parallel equal operations are performed in the IFFT, an increase in the PAPR occurs.
Referring to FIG. 8, to solve such a problem, a discrete Fourier transform (DFT) 12 is first performed on information before a signal is mapped to a sub-carrier in the SC-FDMA. Sub-carrier mapping 13 is performed on a signal spread (or precoded in the same meaning) by the DFT, and the signal subjected to the sub-carrier mapping is converted into a signal in the time axis by performing an IFFT 14.
In this case, unlike the OFDM, the PAPR of a signal in the time domain after the IFFT 14 is not increased so much by the correlation among the DEF 12, the sub-carrier mapping 13 and the IFFT 14, and thus the SC-FDMA is advantageous in terms of transmission power efficiency.
That is, a transmission scheme in which the IFFT is performed after DFT spreading is referred to as the SC-FDMA.
As such, the SC-FDMA has a similar structure to the OFDM, thereby obtaining the signal strength for a multi-path channel, and the SC-FDMA completely prevents the PAPR from being increased through the through the IFFT in the conventional OFDM, thereby enabling the use of a power amplifier. Meanwhile, the SC-FDMA may also be called as DEF spread OFDM (DEF-s-OFDM).
That is, the PAPR or cubic metric (CM) may be decreased in the SC-FDMA. When the SC-FDMA transmission scheme is used, it is possible to avoid a non-linear distortion period of the power amplifier, and thus the transmission power efficiency can be improved in an UE of which power consumption is limited. Accordingly, it is possible to increase a user throughput.
Meanwhile, the standardization of the LTE-A more improved than the LTE has been actively performed in the 3GPP. In the process of standardizing the LTE-A, the SC-FDMA-based scheme and the OFDM scheme competed with each other, but a clustered DEF-s-OFDM scheme that allows non-contiguous resource allocation has been employed.
Hereinafter, the LTE-A system will be described in detail.
FIG. 9 is a block diagram a clustered discrete Fourier transform-spread-orthogonal frequency division multiplexing (DFT-s-OFDM) transmission method employed as an uplink access method in the LTE-advanced standard.
The important feature of the clustered DFT-s-OFDM is that it is possible to flexibly cope with a frequency selective fading environment by enabling frequency selective resource allocation.
In the clustered DFT-s-OFDM scheme employed as the uplink access scheme of the LTE-A, the non-contiguous resource allocation is allowed differently from the SC-FDMA that is an uplink access scheme of the conventional LTE, and thus transmitted uplink data can be divided into several cluster units.
That is, the LTE system maintains a single carrier characteristic in the UL. On the other hand, the LTE-A allows a case in which data subjected to DFT-precoding is non-contiguously allocated on the frequency axis or the PUSCH and PUCCH are transmitted at the same time.